Several independent researchers have postulated that there is a primary dysfunction in early infantile autism in the processing of semantic information, and numerous studies support this hypothesis. However, in none of these studies was semantic or conceptual knowledge tested independently of the use of such knowledge in cognitive tasks; therefore, the precise nature of the hypothesized semantic deficit remains unknown. The research proposed here is directed towards distinguishing between alternative interpretations of the current literature in order to clarify those deficits which are specific to autism. Children diagnosed as early infantile autistic will be compared in a series of experiments to mentally-retarded and normal controls, matched on IQ, mental age, and socio-economic status. To test whether autistic children have structured and organized conceptual and semantic knowledge, similarity judgments for pictures and words will be obtained in the first two experiments. These judgments will be elicited using a matching-to-target procedure. The third experiment will test the subjects' ability to use semantic information in free-recall and cued-recall memory tasks. The final experiment will explore how new concepts and words are learned, to test whether the root of the semantic deficit lies in deficient semantic acquisition processes. Artificially created concepts will be taught and the ability to generalize these new concepts to novel examples will be measured. The major significance of this research will be in providing a clearer understanding of the underlying cognitive deficits associated with autism. This will lead to the development of improved diagnostic tools, and will ultimately direct educational programs to more efficient and successful methods of training language and communication skills to autistic children.